The present invention relates to an electrically insulated coil obtainable by winding around a coil a mica tape which cures at a relatively low temperature and has a long storage life, providing a coil having excellent properties with respect to heat resistance, mechanical characteristics, and electrical insulation characteristics.
Recently, as electric appliances have been of a smaller size, there has been a growing demand for electrically insulated material possessing a higher resistance to heat so that there has been progress in the development of class H insulation continuously operable below 180.degree. C. Heretofore, it was known that a heat-resistant resin for class H insulation includes an aromatic polyimide resin, a silicone resin or the like. The polyimide resin, however, employs a polar solvent so that its use may cause operating and sanitation problems. Furthermore, its final curing temperature is high and it produces water during the curing reaction so that the formation of a minute and elaborate insulating structure requires involved operational conditions. Accordingly, its use has been limited to the preparation of a wire or film. The silicone resin, on the other hand, also has a high final curing temperature, and a cured insulating material has defects in that its mechanical characteristics and electrical insulation characteristics are slightly inferior. As a resin for class H insulation, a resin using as a raw material a maleimide compound is known. Where the maleimide compound is used as the raw material, however, the resin produced by curing has a defect in that it is mechanically brittle and has few practical applications.
As a result of extensive research, the inventors have found that a composition consisting of an epoxy resin, when heat-cured in the presence of an aluminum compound such as aluminum trisacetylacetonate as a curing catalyst, can provide a cured resin possessing favorable mechanical strength and excellent heat resistance. This curing, however, usually requires a long reaction period ranging from several hours to several tens of hours at 150.degree.-250.degree. C. Accordingly, as a result of further research on this point, it has now been found that, when a small amount of a curing accelerator such as a silane compound or a polysiloxane compound, each having a hydroxyl group, is added to a composition system consisting of an epoxy resin, and an aluminum compound, there is provided a curing composition that is stable at room temperature and has a long storage life. Furthermore, it is cured by the aid of a remarkable curing accelerating action at a temperature which is not so high and in a short time to provide a cured resin having no mechanical brittleness and having excellent heat resistance.